A. Field of the Invention
This invention relates to the field of microwave filters.
B. Prior Art
Multicavity band pass waveguide filters are known having two or more cavities in which each of the cavities resonates in two orthogonal modes and are coupled together through a iris in a correct sequence as shown in U.S. Pat. Nos. 3,697,898, 2,999,988 and 2,795,763. This coupling realizes a proscribed response characteristic. The coupling between the modes in each of the cavities is provided by a structural discontinuity or obstacle in the cavity such as a splitting screw. The resonate frequency of each mode within each cavity is adjusted by a respective tuning screw. In this way for a two cavity system, there are required two tuning screws in each of the cavities.
In such prior systems, each of the cavities is non-tunable with an input applied to the circular cross sectional end of one cavity and the output taken from the opposing circular cross sectional end of the last cavity. Coupling between the cavities is provided by an iris positioned in a circular cross sectional separating each pair of cavities. Such two cavity filters have not been tunable because the input and output circular cross sectional ends have not been practically moveable. In such three cavity filters, it is topologically impossible to achieve an equal cavity length for each cavity simultaneously. While dual mode single cavity tunable filters have been known, these filters do not lend themselves to multiple cavity construction.
The tunability of waveguide filters is very important in the field of communications and particularly in the preselection of frequencies for receivers and postselection of frequencies for transmitters.